As shown in the schematic view of FIG. 6, a unidirectional condenser microphone unit (hereinafter, sometimes referred simply to as a “microphone unit”) includes, as a basic configuration, a cylindrical unit casing 10 and an electrostatic acousto-electric converter 20 housed in the unit casing 10.
The unit casing 10 is formed of a metallic material such as a brass alloy, and is provided with a front acoustic terminal 11 on the front surface (the surface on the side directed to a sound source) thereof and a rear acoustic terminal 12 for taking in a velocity component on the side surface side thereof.
The acousto-electric converter 20 includes a diaphragm assembly 21 stretchingly provided with a diaphragm on a diaphragm ring, a spacer ring 22 formed of an electrical insulating material, and an insulating seat assembly 23 formed by fixing a backplate to an insulating seat.
These constituent elements (the diaphragm assembly 21, the spacer ring 22, and the insulating seat assembly 23) are assembled so as to be laminated in sequence with the inner surface of the unit casing 10 being used as a guide, and thereafter are fixed together in the state in which a proper stress is applied by a lock ring 13 engaging threadedly with internal threads in the unit casing 10.
On the rear acoustic terminal 12, a metallic netty member 14 is put from the inside of the unit casing 10 as a shield member for inhibiting extraneous electromagnetic waves (for example, electromagnetic waves emitted from a cellular phone) causing the production of noise from intruding into the unit casing 10.
The metallic netty member 14 is formed by weaving metal wires. Therefore, the wires are connected electrically to each other at contact points of intersections, so that when the metallic netty member 14 is rounded into a cylindrical shape, the contact becomes unstable. If the netty member 14 of this state is inserted into the unit casing 10 and is put on the rear acoustic terminal 12, the netty member 14 comes into contact with the unit casing 10 at unspecified contact points, so that the shield becomes unstable.
Accordingly, in the invention described in Japanese Patent Application Publication No. 2008-166909, as shown in FIG. 6, in the unit casing 10, a coil spring 15, which is preferably gold-plated and is used to push the metallic netty member 14 against the inner surface of the unit casing 10, is provided. Thereby, the metallic netty member 14 is brought into contact with the unit casing 10 at many points, and therefore the electromagnetic shield is made more reliable.
In the above-described conventional art, however, the coil spring 15 is needed, and accordingly the cost increases. Also, in the case where the pushing force of the coil spring 15 against the metallic netty member 14 is insufficient, the contact is still unstable in some cases.
Also, the acousto-electric converter 20 is fixed into the unit casing 10 by applying a stress by using the lock ring 13. However, if the tightening stress is too high, the acousto-electric converter 20 may be destroyed. Inversely, if the stress is too low, noise may be produced by the looseness between parts when the microphone unit is vibrated.
To solve this problem, the tightening torque of the lock ring 13 is controlled by using a tool such as a torque wrench. However, this work requires much time and labor, and therefore is unfavorable in terms of productivity. Also, the lock ring 13 is usually expensive because of being produced by cutting.
For an inexpensive article using no lock ring, the unit casing 10 is made of, for example, aluminum, and the acousto-electric converter 20 is fixed by staking the opening portion on the rear end side of the unit casing 10. However, in this case as well, it is difficult to apply a proper staking force (a stress corresponding to the incorporated part). Further, the productivity is poor because the microphone units are staked one by one by using a staking machine.
Also, for the acousto-electric converter 20, because the constituent elements of the diaphragm assembly 21, the spacer ring 22, and the insulating seat assembly 23 are assembled with the inner surface of the unit casing 10 being used as a guide, a certain degree of clearance is needed between each of the constituent elements and the unit casing 10. However, if the clearance is too large, there occurs eccentricity (off-centering) among these constituent elements, whereby the performance is varied or deteriorated by the change in effective vibration area and the like.
Accordingly, an object of the present invention is to provide a unidirectional condenser microphone unit in which reliable electromagnetic shieldability is provided for a rear acoustic terminal of a unit casing, an acousto-electric converter can be fixed in the unit casing by applying a proper stress, and the eccentricity among the constituent elements of the acousto-electric converter in the unit casing is made as small as possible.